The present invention relates to acicular magnetic iron oxide particles for high-density recording, having high coercive force and excellent erasing characteristic and coated with double coating layers consisting of a Co compound layer as a lower layer and a spinel-type ferrite layer containing Mn-Zn, Ni-Zn or Mn-Ni-Zn as an upper layer, a process for producing the acicular magnetic iron oxide particles, and magnetic recording media having excellent erasing characteristic, produced by coating a substrate with a resin composition containing the acicular magnetic iron oxide particles.
More particularly, the present invention relates to (1) acicular magnetic iron oxide particles coated with double coating layers consisting of a Co compound layer as a lower layer and a spinel-type ferrite layer containing Mn-Zn, Ni-Zn or Mn-Ni-Zn as an upper layer (the particle being hereinafter referred to as acicular magnetic iron oxide particles of the present invention); (2) a process for producing the acicular magnetic iron oxide particles comprising heat-treating a mixed solution having a pH of 11 or above obtained by mixing a water dispersion of acicular magnetic iron oxide particles with at least an aqueous solution of Co salt and an aqueous alkaline solution, at a temperature within the range of 50.degree. to 100.degree. C. to form a Co compound layer on the surfaces of the acicular magnetic iron oxide particles, adding Fe (II) salt and (a) Mn salt and Zn salt, (b) Ni salt and Zn salt or (c) Mn salt, Ni salt and Zn salt to the thus treated solution of pH 11 or above containing the acicular magnetic iron oxide particles coated with a Co compound layer, and heat-treating the thus mixed solution at a temperature within the range of 50.degree. to 100.degree. C. to form on the Co compound layer a spinel-type ferrite layer containing Mn-Zn, Ni-Zn or Mn-Ni-Zn (hereinafter referred to as spinel-type ferrite layer); and (3) magnetic recording media produced by coating a substrate with a resin composition containing the acicular magnetic iron oxide particles of the present invention.
Recently, with advancement of long time recording capacities and lightweight and compact designing of magnetic recording and reproducing devices, the necessity of higher performance of magnetic recording media such as magnetic tapes, magnetic discs, etc., is growing increasingly. Especially, the improvement of recording density chracteristics is of paramount necessity.
Generally, magnetic recording media are produced by coating a substrate with a resin composition containing magnetic particles.
For improving the recording density characteristics of magnetic recording media, it is necessary to enhance the coercive force of magnetic recording media. For attaining this, it is required that the particles of magnetic material used have as high coercive force as possible. This fact is pointed out in, for instance, "Technological Study Reports of The Institute of Electronics and Communication Engineers of Japan" MR77-36, published by The Institute of Elecronics and Communication Engineers of Japan in 1978, which states on page 37: "For increasing the recording density of a magnetic tape, it is necessary to elevate the coercive force of magnetic powder used for the tape."
Presently, so-called Co doped type acicular magnetic iron oxide particles and so-called Co coated type magnetic iron oxide particles are known as magnetic iron oxide particle having high coercive force. These magnetic iron oxide particles have a tendency to be elevated in coercive force as the Co content increases. The Co doped type magnetic iron oxide particles can be produced by first preparing Co-containing acicular goethite particles by adding Co salt to the starting material acicular goethite particles in their preparation reaction and then reducing them to form Co-containing acicular magnetite particles, or, if necessary, further oxidizing the thus obtained magnetite particles to form Co-containing maghemite particles. The Co coated type magnetic iron oxide particles can be obtained by using as precursor particles the acicular magnetite particles obtained by reducing the starting acicular goethite particles or acicular maghemite particles obtained by further oxidizing the magnetite particles, and coating the surfaces of the precursor particles with a Co compound.
Since the magnetic recording media are used repeatedly for a long time, it is also strongly required that they are stable thermally and resistant to change with time and also have excellent erasing chracteristic.
For satisfying these requirements for magnetic recording media, it is necessary that the magnetic properties of the magnetic iron oxide particles used have thermal stability, are resistant to change with time and have excellent erasing characteristic.
The magnetic iron oxide particles and magnetic recording media having high coercive force and excellent thermal stability and erasing characteristic and being resistant to change with time are most strongly demanded at present. The afore-mentioned Co doped type magnetic iron oxide particles and the magnetic recording media produced by using the particles have high coercive force, but on the other hand, they have the problem that the coercive force distribution is too wide due to diffusion of Co ions in crystals, resulting in poor thermal stability and resistance to change with time and bad erasing charactreristic.
This phenomenon is mentioned in "Technological Study Reports of The Institute of Electronics and Communication Engineers of Japan" which states: "Co solid-solution type (doped type) magnetic iron oxide powder is liable to change in coercive force thermally with time, so that it has the serious defect that when a tape is made therefrom, such tape shows poor in print-through and erasing characteristic. Such defect is considered attributable to the fact that Co ions move in crystals even at room temperature."
The Co coated type magnetic iron oxide particles and the magnetic recording media produced by using such particles have high coercive force and are also more stable thermally and resistant to change with time, and more excellent in erasing characteristic than the Co doped type magnetic iron oxide particles. Such fact is noted from "Technological Study Reports of The Institute of Electronics and Communication Engineers of Japan" which states: ". . . the Co epitaxial (Co coated type) magnetic iron oxide powder, because of its double-coated layer structure, is free of these defects and stable thermally and resistant to change with time, and the tape produced by using such magnetic powder has excellent print-through and erasing characteristic."
However, there is no end to the request for improvements of erasing charcteristic in recent technology, and the Co coated type magnetic iron oxide particles and magnetic recording media produced by using such particles can hardly be seemed satisfactory in erasing characteristic, due to wide spread of coercive force distribution.
This fact is referred to in many literatures. For instance, Japanese Patent application Laid-Open (KOKAI) No. 61-17426 (1986) states in its specification: "In the case of magnetic powder using the .gamma.--Fe.sub.2 O.sub.3 particles, it was found that the coercive force distribution widens as the .gamma.--Fe.sub.2 O.sub.3 particles are reduced in size, and there is a tendency that the coercive force distribution is further widened when the particles are coated with cobalt compound layer . . . When the cobalt coated type .gamma.--Fe.sub.2 O.sub.3 particles are reduced in size for attaining high density recording, although a desired coercive force (Hc) is obtained, the produced magnetic powder has an unfavorable coercive force distribution and is poor in erasing characteristic."
The coercive force distribution of Co coated type magnetic iron oxide particles tends to widen as the amount of Co increases, resulting in worse erasing characteristic of the particles. Thus, there is an inverse corelation between the amount of Co and improvement of coercive force distribution.
Under these circumstances, there has been in the industries a strong request for the establishment of a process capable of producing Co coated type magnetic iron oxide particles further improved in erasing characteristic while maintaining high coercive force and the magnetic recording media coated with a resin composition containing such particles.
As a result of extensive studies for producing Co coated type magnetic iron oxide particles further improved in erasing characteristic while maintaining the desired coercive force and the magnetic recording media coated with a resin composition containing the particles, it has been found that (1) acicular magnetic iron oxide particles coated with double coating layers consisting of a Co compound layer as a lower layer and a spinel-type ferrite layer containing Mn-Zn, Ni-Zn or Mn-Ni-Zn as an upper layer, obtained by first preparing a mixed solution with a pH of 11 or above by mixing a water dispersion of acicular magnetic iron oxide particles with at least an aqueous solution of Co salt and an aqueous alkaline solution, heat-treating the thus mixed solution at a temperature of 50.degree.-100.degree. C. to form a Co compound layer on the surfaces of the acicular magnetic iron oxide particles, then adding Fe (II) salt and (a) Mn salt and Zn salt, (b) Ni salt and Zn salt or (c) Mn salt, Ni salt and Zn salt to the mixed solution of pH 11 or above containing the acicular magnetic iron oxide particles coated with a Co compound layer, and heat treating the thus mixed solution at a temperature of 50.degree.-100.degree. C. to form on the Co compound layer a spinel-type ferrite layer containing Mn-Zn, Ni-Zn or Mn-Ni-Zn, and (2) magnetic recording media produced by coating a substrate with a resin composition containing the thus obtained acicular magnetic iron oxide particle have high coercive force and excellent erasing charcteristic. The present invention has been attained on the basis of said findings.